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Tautology refers to redundancy, repetition, and circular reasoning within an argument or statement.

In logic, a tautology is a statement that is true regardless of the truth-values of its parts.

For example, the statement "All crows are either black, or they are not black," is a tautology because it is true no matter what color crows are.

The opposite of a tautology is a contradiction, which is a statement that is always false.

In linguistics, a tautology is often a fault of style. It was defined by Fowler as "saying the same thing twice". For example, "three-part trilogy" is tautologous because a trilogy, by definition, has three parts. "Significant milestone" and "significant landmark" are also if less obviously tautologous, because milestones and landmarks are again significant by definition (could one imagine an "insignificant landmark"?).

See also: Pleonasm